Geological setting of silcretes of Apt (South-East of France)
The studied samples originate from a sandstones series of Apt (South-East of France, Figure
1)1,2. A clastic deposit of quartz sands occured during Mid-Cretaceous in the Alpine Sea. The
sedimentation of quartz was proven to be marine by the presence of synsedimentary
glauconite. This sedimentation was followed by a diagenetic cementation of quartz, leading to
the formation of “glauconitic sandstones” (sample labeled "DS" for marine diagenetic
sandstone, Figure 1). During Upper-Cretaceous, tectonic constraints formed the Durance
uplift, which brought the glauconitic sandstones in an emerged position where they were
subjected to weathering under a wet tropical climate2. In the upper part of the series, the
weathering of quartz marine cements and glauconite led to the formation of "rubefied sands"
composed of the primary clastic quartz associated with kaolinite, goethite and hematite
formed during weathering. The circulation of meteoric waters in these newly porous sands led
to the formation of siliceous crusts by precipitation of quartz in the pores. By measuring the
melting temperature of the water trapped in the inclusions of quartzitic cement3, it was proved
that the water from which the quartz precipitated was low temperature (<40°C) fresh water.
This result shows that, contrary to the previous marine diagenetic cementation, these
silicifications occured in a continental environment. In the series, two kinds of continental
silicification may be distinguished : pedogenic silcretes (sample labeled "PS", Figure 1) which
were formed in soils at the top of the series, and groundwater silcretes (samples "GS1", "GS2"
and "GS3", Figure 1) formed in the aquifer deeper in the weathering profile1,3,4.
Comparison of the three techniques for measuring Si isotopes (IRMS, MC-ICP-MS,
SIMS). In order to evaluate the benefits of SIMS for silicon isotopic measurements, we
compare in this section the analytical performances of SIMS with the methods used up to now
for measuring silicon stable isotopes in terrestrial samples. The Table SI 1 synthesizes the
analytical characteristics of the three methods (IRMS, MC-ICP-MS, SIMS). By IRMS, any
kind of bulk sample (total rock, set of diatoms, water...) can be analyzed. The 30Si is
measured with a good precision (±0.2‰ (2which has been recently improved (±0.1‰
(25This method is however highly dangerous due to the use of hazardous gazes as F2 or
BrF5. The MC-ICP-MS method allows, as well as IRMS, the analysis of any kind of bulk
sample. Only measurements of the masses 28Si and 29Si are reliable, due to the large
interference of 14N16O on mass 30Si. 30Si has to be calculated from 29Si with an empirical
relation6 (Table SI 1). Cardinal et al. (2003)7 however reached a very good precision on 29Si
(±0.08‰ (2by using an external standardization method. Contrary to the previous
methods, SIMS does not perform analyses on bulk samples, but on in situ individual grains
larger than ~50 µm in diameter. The sample analyzed volume is estimated to be ~4500 µm3. It
is possible to measure different types of grains that constitute a single rock whereas IRMS and
MC-ICP-MS would only give a mean value. However, water samples cannot be measured by
SIMS. In term of analytical precision, SIMS measurements are less precise (±0.75‰ (2
Table SI 1 than IRMS and MC-ICP-MS. In term of accuracy Table SI 1, the comparison of
the three methods is more delicate, since no standard with a certified value exists. To our
knowledge, three 30Si value of Rose Quartz have been reported : (1) 30Si = 0 ±0.2‰8
(authors reported exactly that measured 30Si/28Si ratio of Rose Quartz equal 30Si/28Si ratio of
NBS28, which means that 30Si of Rose Quartz was 0 ±0.2‰ (2; (2) 30Si = -0.3 ±0.2‰9 ;
and (3) 30Si = -0.28 ±0.2‰10. Ding (1996)11 used a value of 30Si = -0.3‰ but did not report
the measurements. All these measurements where performed by IRMS, but one has to
remember that during sample preparation (Table SI 1) fractionations may occur at several
steps. The accuracy of MC-ICP-MS was not checked by using the Rose Quartz. The two
methodological studies using this instrument6,7 used their own standard and compared the
value with the value obtained by IRMS measurements (Table SI 1). By SIMS, we obtained a
Rose Quartz value slightly higher than IRMS (30Si = +0.26 ±0.75‰ (2)) Table SI 1.
However, if one takes into account the error bars, we can consider that the SIMS
measurements are rather accurate in comparison with IRMS.
We can conclude that, in spite of a lower precision, the analysis of silicon isotopes by SIMS
provides fast and reliable measurements on in-situ individual grains. However, the
intercalibration of the different techniques suffers of a lack of certified standard that should be
developed in the future.
Si isotopes fractionation during chemical or biochemical precipitations of Si : state of
the art
Orders of magnitude of Si isotopes fractionation are reported for chemical or
biochemical precipitations of Si in water of known 30Si. Chemical precipitation experiments
have shown11-13 that silica and clay were 0.3‰ to 3.8‰ more negative than the final water.
These results confirm the principle of kinetic isotopic fractionation, according to which
dissolved H428SiO4 tends to precipitate preferentially, leaving the residual solution enriched in
H430SiO411. Moreover, if the precipitation keeps on going in a closed system, the distillation
following the Rayleigh equation indicates that both dissolved and precipitated Si will get
progressively 30Si enriched11. The values reported for these simple chemical precipitations are
much smaller than the ones observed here for the mQII and MQII of the silicified samples. In
the case of biochemical precipitations, an additional fractionation could occur through a
H4SiO4 assimilation by living organisms14. In growth experiments of plants and diatoms, large
fractionation were not observed since biogenic silica was 0.6‰ to 1.6‰ more negative than
the initial water8,12,13. However, it has been suggested by in situ sampling that marine sponges
fractionate Si isotopes in a larger range (-3.8‰)14. Thus, neither chemical nor biochemical
precipitations seem to be responsible for the negative values measured in this work.
An other parameter susceptible to induce fractionation during precipitation is
temperature. However, in growth experiments of diatoms in batch culture under a temperature
gradient (12°, 15° and 22°C)8, no temperature effect was observed. Temperature is thus
unlikely the key parameter of the fractionations that we observe in the quartz that all
precipitate at surface temperatures.
Si isotopes fractionation during silicification processes
The difficulty of suggesting an explanation for our measured 30Si values stems from
the fact no Si isotopes fractionation measurements during quartz crystallization (  quartz- sol )
exist and of course the solutions in which mQII and MQII crystallized during the Cretaceous
era can no longer be sampled. It may be however interesting to focus on the physicalchemical processes that control the precipitation of various types of quartz. It is known that
the solubility of SiO2 polymorphs increases with decreasing density, structural order15 and
particle size16. Thus, in our case, the solubility of mQII should be higher than that of MQII.
According to Thiry (1997)4, the mechanisms of silicification follow the same pattern as those
of solubility and the SiO2 cements are formed by several precipitation/dissolution steps. Thus
the most soluble minerals (here mQII ) precipitate first. The less soluble minerals (MQII) then
precipitate from solutions locally fed in Si by the dissolution of the most soluble minerals
(mQII). The fractionations  between precipitated quartz and solution can then be defined by
the two following equations:
mQ II sol1  δ30Si mQ II  δ30Sisol1
(1)
and
MQ II sol2  δ30Si MQ II  δ30Sisol2
(2)
where sol1 is the initial solution and sol2 the solution in which dissolved Si comes from
the dissolution of mQII. Assuming that dissolution does not fractionate the Si isotopes (see
next section), then δ30Sisol2  δ 30Si mQ II and  MQ II  sol2 can be calculated for samples PS, GS1,
GS2 and GS3 using equation 2. The obtained values of  MQ II  sol2 go from -1.3‰ to -1.8‰
(with an average of -1.5‰). The range of variation is narrow and within the analytical
precision (±0.75). This result shows that whatever the form of quartz (in desiccation cracks
for PS sample or overgrowths for GS samples) its crystallization at surface temperature
fractionates the Si isotopes. The order of magnitude of fractionation by quartz is equivalent to
the above mentioned values measured in the laboratory for silica, clays and biogenic silica. As
fractionation is most likely controlled by kinetic effects, the mean value found for
 MQ II  sol2  1.5 ‰ is correct whatever the size of the quartz sample (micro or
macrocrystalline). In addition, this value is coherent with the mQII of DS sample which also
precipitated at low temperature (30-40°C)3. The 30Si of mQII (0.2 ± 0.6‰), likely
precipitated from sea water with a 30Si value around 1.7‰, which is in the range of sea water
30Si values ( +0.6 to +1.7‰, 69 measurements)9.
With this value of  quartz-sol  1.5 ‰ and the assumed precipitation/dissolution
processes discussed above, we can calculate (equation 1) the 30Si values of the initial
solutions of each silicification level and suggest different possible explanations for the
measured mQII and MQII 30Si values. Both for pedogenic silicification (PS sample) and GS1
groundwater silicification, the values found for 30Sisol1 (-0.3‰ and -0.1‰ respectively) can
be explained by the presence of a Si solution fed by the dissolution of igneous rocks and a two
steps quartz crystallization process (mQII followed by MQII) involving twice the  quartz- sol
fractionation. On the contrary, in the case of GS2 and GS3 groundwater silicifications, the
solutions are 30Si depleted (-1.4‰ for GS2 and -2.6‰ for GS3). These values can be
explained by considering that groundwater silification takes place through a vertical dynamic
evolution of the aquifer. A vertical superposition of three groundwater silcretes could
correspond to successive deepening levels of the watertable4. Chronologically, GS1 would
correspond to the first silicification level, GS2 to the second and GS3 to the third. In this
scenario, each new silicified level would be fed by water that passes through the above
unsaturated zone and dissolves the remaining mQII of the above silicified levels. This matches
the observed isotopic data since 30Sisol of GS2 (-1.4‰ ) is close to mQII of GS1 (-1.6 ±
1.1‰) and 30Sisol of GS3 (-2.6‰ ) is close to mQII of GS2 (-2.9 ± 1.3‰). In this context, it
would not be surprising to find elsewhere on Earth, other silicification areas with 30Si values
even more negative than the ones found here. Whatever the scenario, it may be concluded that
very negative values of 30Si (at least -5.7 ± 1.9‰) can be achieved by successive
dissolution/precipitation of SiO2 polymorphs of decreasing solubility. Moreover, the above
results show that in continental environments, certain groundwaters may also be counted as
30
Si depleted pools.
Arguments in favour of absence of Si isotopes fractionation during dissolution of quartz.
It is very unlikely that incongruent dissolution between 28Si, 29Si, and 30Si occurs during
dissolution of quartz. Dissolution experiments of other types of minerals have been tested
with the isotopes of strontium (86Sr and 87Sr). Fractionation of Sr isotopes was observed
during dissolution of biotite. It was attributed to the heterogeneity within its crystallographic
structure, the biotite being a phyllosilicate. On the opposite, no fractionation was observed
during the dissolution of labradorite, a plagioclase (feldspar) which has a tectosilicate
structure, as quartz does17. Finally, in a recent study of lithium fractionation during
continental weathering processes, the authors also consider that incongruent dissolution of 7Li
and 6Li in a single primary mineral is unlikely18. However, the above observations only
concern trace elements whereas Si is a major element of quartz. Thus, the above hypothesis
should be verified in future laboratory works.
Equation of isotopic budget of Si
Q weatheredSi δ 30 Si igneousrocks 
Q
poolSi i
poolSi i
δ 30 Si poolSi i
where Q weatheredSi I is the amount of weathered Si from igneous rocks and δ30 Siigneousrocks its
mean isotopic composition, and Q poolSi i is the amount of Si locked up in a continental pool i
(i representing the various Si pools -except igneous rocks- presented on Figure 3) and
δ 30 Si poolSi i its mean isotopic composition.
For the estimation of the weathered Si locked up in silicified rocks, the following were
assumed : (1) the δ 30Si of weathered primary minerals is -0.3‰, (2) the δ 30Si of rivers is
+1.1‰5,9, (3) the δ 30Si of quartz, precipitated at surface temperature, is -3.3‰ (mean value of
our different types of quartz), and (4) the amount of biogenic Si, Si in clays and Si in
groundwaters are neglected.
Table SI 1 : Comparison of analytical characteristics of IRMS, MC-ICP-MS and
SIMS for stable silicon isotopes measurements of terrestrial samples.
Table SI 1 part 1/2
Type of
analyzed
samples
Sample preparation
(related problems)
Instrument
Nier MS
(1)
SIMS
MC-ICP-MS
IRMS
Purification of Si02 and
fluorination
Bulk soild
sample
Water
sample
Bulk solid
samples
Water
samples
Individual
grains of
quartz
(Use of hazardeous
F2 and BrF5;
Potential isotopic
fractionation and
contamination during
sample preparation)
Dissolution with HF
and dilution in
HCl solution
Thin section of
rocks
or
polished grains
Resolving power M/M
(possible interferences)
Reliable
Measured
Isotopes
nr
(28SiF3 : COF3)
28
Si 29Si 30Si
MAT 251 EM
(2a)
(2b)
nr
(28SiF3 : COF3)
28
Si 29Si 30Si
VG-PrismMAT 252
(3a) (3b) (3c)
(3d) (5)
nr
28
Nu Instrument
Static mode
(4)
Nu Instrument
Dry plasma
dynamic mode
(5)
CAMECA
1270
Static mode
(6)
nr
(28Si : 14N2)
Si 29Si 30Si
Measured
Ions
SiF3+
30Si : SiF3+
29Si : SiF4+
SiF4+
28
28
Si 29Si
29
(30Si : 14N16O)
300
(28Si : 14N2)
28
28
Si 29Si
(30Si : 14N16O)
29
Si+
Si+
Si+
Si+
28
5000
28
Si 29Si 30Si
30
SiSi-
(1) Douthitt (1982) 19; (2a) Ding et al. (1996) 11; (2b) Ding et al. (2004) 5; (3a) De La Rocha et al.
(1996) 20; (3b) De La Rocha et al. (1997) 8; (3c) De La Rocha et al. (1998) 21; (3d) De La Rocha et al.
(2000) 9; (4) De La Rocha et al. (2002), De La Rocha et al. (2003) 6,14; (5) Cardinal et al. (2003) 7; (6)
this study. nr signifies that the value was not reported.
Table SI 1 part 2/2
Internal analytical
precision (1 better
than
Reproducibility
Standard deviation (2)
(n measurements)
Accuracy
nr
30Si : 2= 0.3‰ to 0.6‰ (n=nr)
Rose Quartz used as normalizing standard
MAT 251 EM
(2a)
(2b)
30Si : 0.1‰ (2a)
nr (2b)
30Si : 2= 0.2‰ to 0.6‰ (n=4)
(2a)
30Si : 2 = 0.1‰ (n=8) (2b)
nr
VG-PrismMAT 252
(3a) (3b) (3c)
(3d) (5)
nr (3)
29Si : 0.035‰ (5)
30Si : 2 = 0.2‰ (n=nr) (3)
29Si : 2 = 0.18‰ (n=23) (5)
(3a) Rose Quartz used as normalizing standard
(3b) 30Si/28Si Rose Quartz = 30Si/28Si NBS28
(3c) nr
(3d) Rose quartz : 30Si = -0.3‰ ± 0.2 (n=nr)
Nu Instrument
Static mode
(4)
nr
29Si : 2 = 0.2‰ to 0.04‰ (n=10)
29Si of diatoms : 0.54‰ by MC-ICPMS and
0.43‰ by IRMS (n=nr)
Instrument
SIMS
MC-ICP-MS
IRMS
Nier MS
(1)
29Si : 2s = 0.24‰ (n=23) by
standard-sample bracketing
Nu Instrument
Dry plasma
dynamic mode
(5)
29SI : 0.077‰
CAMECA
1270
Static mode
(6)
30Si : 0.08‰
29Si : 2s = 0.08‰ (n=23)
by external standardization
30Si : 2= 0.75‰ (n=45)
29Si of in-house std : -5.05‰ by MC-ICPMS and 5.39‰ by IRMS (n=23)
Rose quartz : 30Si = +0.24‰ ± 0.75 (n=11)
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